Deposition of solid phases on substrates by decomposition of volatile or gaseous compounds which contain the solid phase elements is generally referred to as chemical vapor deposition. If this deposition takes place in the open pores of a porous substrate or in the cavities of a porous structure, then it is often referred to as chemical vapor infiltration. Chemical vapor deposition (CVD) and chemical vapor infiltration (CVI) allow a densification of structure or, when the porous structure consists of fibers, an introduction of a matrix and, with this, the production of composite, strengthened fiber materials. Both chemical vapor deposition as well as chemical vapor infiltration are extremely complex processes.
In chemical gas phase infiltration, the volatile or gaseous starting compounds must be transported into the depths of the pores before conversion to the solid matrix. If decomposition of the volatile or gaseous starting compounds, and formation of the solid phase, occurs in the gas phase and on the surface of the porous structure in or near the entrances of the pores, the pores become clogged. The pores are then not filled, which defeats the whole aim of the process.
Various conventional methods for CVI/CVD processing are known. Procedurally, the simplest methods to perform are isobaric and isothermic chemical vapor infiltrations. In these conventional methods, the entire process space exists at constant temperature and pressure. Here, however, only very low pressures or partial pressures of educt gases can be used, when necessary with addition of inert or dilution gases, so that extremely long infiltration times are required. Optimal or maximal pore filling is generally thought to be possible only at extremely slow deposition or infiltration rates.
In order to successfully bring about infiltration, low pressures, in particular, low partial pressures of reactant gases, have been used. The pressures realized under the conditions of industrially applied chemical vapor infiltration are at least one to two orders of magnitude below normal pressure. Starting compounds are partially mixed with inert gases so that their partial pressures, and with it their deposition rates, can be further lowered. Due to the low partial pressures, extremely long infiltration times of up to several weeks are required.
In vacuum pressure pulsation methods, the process pressure is continually pulsed, presumably to support diffusion. Significant disadvantages of this method include the cost of the apparatus as well as the filtration times, which are still very long.
Another well-known method is the temperature gradient method, described for instance in U.S. Pat. No. 5,411,763 and U.S. Pat. No. 5,348,774. In this method, heat is removed from the side of the porous substrate facing the process gas stream by suitable measures, for example by cooling by the stream. The side of the porous substrate opposite to the gas stream is adjacent to a heating element. In this way a temperature gradient crucial to the method is established normal to the surface of the substrate. The surface temperature on the cold side is adjusted with the gas stream such that no, or at least very little, deposition takes place. This avoids narrowing of the pores in the region. A disadvantage of this method is the very high gas throughput necessary for cooling. The low yield of deposited material entails long production times. Much equipment is needed for the heating.
DE 41 42 261 teaches CVI/CVD methods in which the gas is streamed through the porous substrate on the basis of forced convection whereby a pressure gradient is established. The infiltration time can be kept relatively short. After a certain level of pore filling however, the streaming through of the porous structure becomes more difficult.
From U.S. Pat. No. 4,580,524, teaches a CVI/CVD method whereby temperature and pressure gradient techniques are combined with one another. In this way relatively short production times can be achieved. The disadvantage of such a method is the complicated reactor construction.
Huttinger (U.S. Pat. No. 6,197,374 B1) describes an isothermal method for chemical vapor infiltration of porous refractory materials. The infiltration takes place under isobaric conditions, e.g. the porous structure to be infiltrated is streamed with a gas in a reaction zone, but is not subjected to a flow through such that an appreciable pressure gradient is formed. In this patent, the gas pressure or the partial pressure of an educt gas contained within the gas and the persistence of the gas in the reaction zone are adjusted for the prescribed temperature in the reaction zone such that in the porous structure a deposition reaction in the pressure and partial pressure region of the saturation adsorption of the gaseous and volatile compounds forming the solid phase exists. The gas pressure or the partial pressure of an educt gas contained within the gas and the persistence of the gas in the reaction zone are adjusted for the prescribed temperature so that the transformation of the educt gas is limited such that in the flow through of the reaction zone no more than 50% of the solid-forming elements introduced into the educt gas are deposited in the porous structure. In addition, the porous structure is subjected to flow-through gas linearly from the bottom to the top through apertures of substantially identical width from 1 to 50 mm. In this process, substantially higher pressures and partial pressures of the educt gas are set compared to conventional method. These higher pressures are said by the Huttinger patent to be higher than those of known isobaric, isothermal methods, in order to enable high or the highest possible deposition rates.
The Huttinger patent teaches that in order to simultaneously achieve good pore filling while applying high pressure at the same time, very special reaction control is required, and the choice of starting compound is of special importance. Methane or natural gas alone often find preferred implementation in the chemical vapor infiltration processes of carbon due to the fact that they are inexpensive.
U.S. Pat. No. 6,197,374 B1 teaches that the combination according to its invention of high pressures on the one hand and low deposition rates on the other has the effect that chemical vapor deposition according to the method of its invention attains a combination of high production speed and a high extent of pore filling. The patent teaches further that according to its invention the gas flow to which the porous structure is subjected may contain a significant portion of inert or dilution gas, e.g. nitrogen, argon, etc., but that preferably no inert or dilution gas is added to the gas. Natural gases which by nature contains a small amount of inert or dilution gas can be used, but no additional inert or natural gas should be added to lower the partial pressure of the starting materials.
The disclosure of the Huttinger patent may be summarized as follows: “Decisive for the method according to the invention are therefore the right choice of starting materials, high pressures and especially high partial pressures of the starting materials and low temperatures”. U.S. Pat. No. 6,197,374 B1, column 8, lines 8-11.